ASA 127th Meeting M.I.T. 1994 June 6-10

4pUW10. Adaptive estimation of acoustic normal mode amplitudes.

The normal mode amplitudes and their second-order statistics are useful in
understanding sound propagation in the medium and for matched-mode processing
(MMP) and matched-field tomography (MFT). Standard modal beamforming techniques
introduce a least-squares error criterion to compute the modal amplitude
estimates. This work uses a method of adaptive estimation which is capable of
processing both coherent and incoherent modes. The new approach is
fundamentally different from other modal estimators (e.g., MMP) because it is
data-adaptive and maximizes SNR against an ambient noise background instead of
minimizing squared error in the estimate without regard to the noise. The new
methods are studied using simulations which include coherent and incoherent
modes and a realistic ocean noise model. The performance of the estimators is
evaluated with respect to the following criteria: (i) orthogonality of the
sampled mode shapes, (ii) power level and spatial structure of the noise, (iii)
mode coherence, (iv) modeling assumptions (number of modes to estimate), and
(v) the presence of multiple modal sources. The new methods perform
significantly better than least squares in high noise environments and in
situations where the sampled modes are not orthogonal.